Abstract

Gram‐positive bacteria of the genus Streptomyces are major inhabitants of soil and grow as branching hyphae, strikingly like the filamentous fungi. Whereas their mycelium
is multicellular, dispersal and long‐term survival are achieved by unicellular spores. Spore formation is part of a complex
developmental cycle involving dedicated reproductive aerial hyphae. The long apical cells of the aerial hyphae are divided
into chains of tens of prespore compartments, which in turn mature into thick‐walled spores with condensed nucleoids and low
metabolic activity, primed for dispersal and survival. Aerial hyphae and spore surfaces are coated with a proteinaceous fibrous
sheath. This highly hydrophobic sheath is required for aerial hyphal emergence, and may aid spore dispersal and interaction
with various interfaces. When exposed to appropriate environmental conditions, spores rehydrate and swell. Cell polarisation
is required for the emergence of a germ tube, which develops into a mycelium by apical growth and side‐branch formation.

Key Concepts:

Streptomyces spores are distinctly different from bacterial endospores.

Schematic view of spore formation in Streptomyces. The vegetative mycelium is represented by hyphae growing down in the medium. On the surface of the colony, aerial branches emerge and extend away from the medium and into the air. Initially, the aerial hyphae form occasional crosswalls, similar to the crosswalls observed in vegetative hyphae. After extension into the air has ceased, the apical cell of the aerial hyphae is segmented into prespores by multiple and regularly spaced sporulation septa. Maturation and rounding up of the prespores result in the final chain of spores, ready for dispersal in the environment.

Schematic view of sporulation septation and spore wall formation. (a) Aerial hyphae are encased within a fibrous sheath (FS), outside of the hyphal wall (HW). (b) Sporulation septa (SS) grow inwards and have a double annular appearance. (c) Thickening of the wall at the sporulation septa is the first stage of spore wall (SW) formation. (d) A uniformly thick spore wall is assembled on the inside of the original hyphal wall. (e) Spore maturation includes rounding up to an ovoid shape and detaching from each other. The fibrous sheath and the original hyphal wall tend to break up to release the spores. Some studies have reported a separation of the spore wall into distinct layers, as indicated in panel (e), but the significance of this observation remain unclear. Redrawn from Wildermuth and Hopwood .

Figure 4.

Micrographs showing different stages of spore germination in S. coelicolor. From left to right, the images show a phase‐bright spore, a spore that has become phase‐dark and partially swollen, a swollen spore in which the cell polarity determinant DivIVA (here visualised using a DivIVA‐EGFP fusion protein) has assembled at a specific location, and finally, a spore with emerging germ‐tube with DivIVA‐EGFP at the apex. Scale bar: 2 μm. Reproduced with permission from Flärdh ().

Kodani S,
Hudson ME,
Durrant MC et al.
(2004)
The SapB morphogen is a lantibiotic‐like peptide derived from the product of the developmental gene ramS in Streptomyces coelicolor.
Proceedings of the National Academy of Sciences of the USA
101:
11448–11453.